The present invention relates to extrusion dies for forming thin-walled honeycomb structures from extrudable materials. More particularly, the invention relates to extrusion dies offering improved utility for the extrusion of thin-walled high-cell-density ceramic and metallic honeycombs.
In the manufacture of ceramic honeycombs, particulate mineral batch materials are dispersed in an appropriate vehicle to form a plasticized powder batch, and the batch is forced through a honeycomb die to provide extruded green bodies of complex honeycomb shape which are then dried and fired. Thin-walled ceramic honeycomb structures thus produced display utility in a variety of applications. For example, such structures are used as substrates for the support of catalysts in automotive exhaust gas treatment systems, as well as for other catalyst carriers, filter bodies, and thermal regenerators or heat exchangers. Metallic honeycombs of similar configuration have been used as gas heaters.
As the art of honeycomb extrusion has advanced, honeycombs with finer and finer cell structures and thinner cell walls have been developed. The production of these finer honeycombs requires that extrusion dies with finer structure be used. Dies used for the extrusion of ceramic honeycombs are typically machined metal plates or blocks having shallow, criss-crossing and interconnecting discharge slots on the downstream or outlet face of the die from which the plasticized batch emerges and through which, during emergence of the batch, the webs or sidewalls of the cells of the honeycomb structure are formed. To supply the batch material to the discharge slots, feed holes communicating with the slots are provided in the opposite or inlet face of the die.
To provide finer honeycombs with higher cell densities (more cells per unit area) and thinner cell walls, the discharge slots and feedholes must of course be formed closer together and be smaller in size. For advanced ceramic honeycomb products, the objectives are to achieve cell densities exceeding 600 cells/in.sup.2 and cell walls below about 200 .mu.m in thickness.
To achieve these dimensions, slot and feedhole machining to very close tolerances is required. Meeting such tolerances has required the use of non-traditional machining processes such as electrochemical machining (ECM) and wire electrical discharge machining (EDM). EDM is the preferred process for generating the discharge slots and ECM is typically used for producing the precise arrays of small feedholes needed to supply the discharge slots.
In the prior art, extrusion dies for the manufacture of ceramic honeycombs have been formed of tool steels or stainless steels. Stainless steels are harder to machine but offer significant advantages for honeycomb die fabrication because they offer a corrosion resistant medium which can withstand relatively high stresses and attack by aqueous media. In addition, many stainless steels can be wear-coated with hard surfacing materials such as carbides and nitrides. Such coatings significantly enhance the ability of the die to resist wear from the abrasive ceramic powder batch materials extruded therethrough.
The difficulty of shaping these very hard stainless steel materials has led to the suggestion of alternative extrusion die manufacturing strategies, particularly where complex feedhole shapes are desired. Thus U.S. Pat. No. 5,308,556, for example, discloses a method of forming an extrusion die from a powder, wherein a powder preform for the die, typically of ceramic composition but optionally of metal, is at least partially machined while in a porous and unconsolidated (green or chalk-hard) state. The shaping of consolidated die blank materials is also mentioned, although specific materials useful in that procedure are not actually described or discussed.
Unfortunately powder-formed dies made as described in the above patent have not yet been proven for use in the production of high-cell-density, thin-walled honeycomb structures. One problem with the described approach is the difficulty of maintaining high dimensional precision in feedholes and/or discharge slots made in unconsolidated materials during the high-shrinkage process of consolidating them to useful densities. Thus the materials of choice for the fabrication of advanced honeycomb extrusion dies are still wrought stainless steels and tool steels.
Descriptions of the use of EDM and ECM for the fabrication of stainless steel extrusion dies are found in the patent literature. U.S. Pat. Nos. 5,320,721 and 5,322,599, for example, describe the application of ECM processes to the machining of die feedholes, while U.S. Pat. No. 4,527,035 documents the application of wire EDM to the machining of discharge slots in the outlet faces of the dies.
In principle, the finer hole and slot dimensions needed for advanced honeycombs can be reached with ECM and EDM machining techniques. In practice, however, the resulting dies do not demonstrate the expected extrusion performance. Forming defects including missing webs (interruptions in the formation of the cell walls of the honeycomb) and swollen webs (wall segments of excessive thickness), are often observed, as is unacceptable twisting or turning ("bowing") of the extruded material as it exits the extrusion die. In general, these defects are usually attributed to defects in design or finish of the extrusion dies.
It would be desirable to develop a die or die machining procedure for making dies for the extrusion of very fine honeycomb structures which would permit the extrusion of honeycombs with thinner walls and/or higher cell counts at yields as high or higher than present honeycomb extrusion processes.
It would also be desirable to develop a die which would permit the extrusion of less advanced ceramic honeycombs at higher yields and in higher quality. Yet, any material selected for this application would have to be sufficiently strong to handle the relatively large extrusion pressures required for fine honeycomb extrusion, and sufficiently durable and wear resistant to resist the abrasive effects of presently used ceramic powder batches.